He ditches because he's bored. Lip is smarter and better at anything academic so smoking and hooking up with girls makes more sense than wasting time in school.

The Gallagher family invites trouble with their unruly behavior and then it's up to Lip to make some money, fix something or figure out a plan to get them out of out hot water. Without the family (or the entire troubled neighborhood), Lip could excel at anything. Heck, he could take over the world with the right motivation.

Military

Lip's brother, Ian, wouldn't like this. Ian desperately wants to go to West Point and has been in the Army ROTC at school for years. With his intellect, any military unit could groom Lip into a brilliant leader. No country could counter Lip's military intelligence. He could make his way to four-star general, and maybe later on in life, a spot as President of the United States.

Internet

Figuring out coding and hacking wouldn't be a problem for someone as smart as Lip. Slowing the Internet or holding it hostage would definitely be a possibility. For what purpose? Whatever purpose Lip deems necessary.

Drug kingpin

Lip isn't an evil person. But he has loose morals. He's a Gallagher. Running drugs for big profit (we're talking multi-millionaire or billionaire) isn't out of the question. After all, nobody figured Walter White on Breaking Bad for a boss.

Revolution

Put Lip in an unorganized country or communist territory and he might very well lead them. If anybody could figure it out, Lip would.